1. Field of the Invention
The present invention (related to a bipolar with an eight-zeros substitution (B8ZS) and a bipolar with a six-zeros substitution (B6ZS) coding circuit used in a digital data communication system. More particularly, it relates to B8ZS.multidot.B6ZS coding circuit used in a digital data multiplexing system which provides a Z8ZS coded signal or B6ZS coded signal at a same timing, and can be formed by a simple circuit construction.
2. Description of the Related Art
In a digital communication system, for example, in a telephone communication system, data transferred in digital communication networks is multiplexed and converted into bipolar signals having three states: a positive logical "1", zero, and a negative logical "1". At a reception side, the received data is converted into unipolar signals having two states: a logical "1" and zero, and demultiplexed. The received bipolar signal is used for extracting a clock signal. Namely, the received bipolar signal is sent to a tank circuit and the clock signal is generated in response to a signal level change of the received bipolar signal. If the bipolar signal comprises consecutive zero data, the signal level is not changed while the consecutive zero data continues and a reasonable clock generation is not carried out, and accordingly, a circuit in a transmission side compulsorily generates a violation signal having a predetermined level change if the consecutive zero data is continued. In a first order group, using a 1.544 Mbps data transmission speed, of the digital communication network, the violation signal is generated when eight consecutive zeros are continued. In a second order group, using 6.312 Mbps of the digital communication network, the violation signal is generated when six consecutive zeros are continued. A B8ZS coding circuit and a B6ZS coding circuit are used to obtain the above violation signal generation. Frequently, a B8ZS.multidot.Z6ZS coding circuit, which is incorporated with the B8ZS coding circuit and the B6ZS coding circuit and is commonly used for the B8ZS coding or the B6ZS coding, is provided in repeater stations and/or terminal equipment in the digital telephone communication system.
However, in a prior B8ZS.multidot.B6ZS coding circuit, a time at which the generation of the violation signal for the eight consecutive zeros detection is started differs from that of the six zero detection. This time difference is a cause of a cumbersome data processing at the reception side. In addition, the prior B8ZS.multidot.B6ZS coding circuit suffers from a complex circuit construction.